Certain motor vehicles, such as medium duty trucks for example, are capable of towing various types of non-fifth-wheel trailers. Such a towing vehicle has a trailer hitch at the rear, a pintle- or ball-type hitch for example, and such a trailer has a coupling at its front that releasably connects to the vehicle hitch.
Regardless of the particular trailer type or model, a trailer's electrical equipment will include various exterior lamps, including lamps capable of signaling a stop, a right turn, and a left turn. The right turn and left turn lamps are also used for hazard warning. Additional lamps that are typically present include: clearance, side marker, and identification lamps; and tail and license plate lamps. For placing such lamps under the control of the correct circuits in the towing vehicle, the trailer electrical system comprises a connector plug, typically forming a termination for one end of a multi-conductor cable, that mates with a receptacle socket in the towing vehicle.
Trailers that are equipped with certain types of brakes have circuits that require proper connection with circuits in the towing vehicles, and those connections occur through mated connector plugs and receptacle sockets.
A trailer that has electric trailer brakes requires electric current from an electric trailer brake controller in the towing vehicle. The electric trailer brake controller may be coupled with the vehicle service brake system to apply the trailer brakes in correlation with how hard the driver is applying the service brakes and/or have a separate actuator that allows the driver to apply the trailer brakes independently of application of the vehicle service brakes. The current to the trailer brakes passes through mated terminals in the plug and socket in an amount correlated with the how hard the driver is applying the service brakes in the vehicle, or optionally how hard the driver is applying the trailer brakes via the manual actuator of the trailer brake controller.
A trailer that has air-actuated trailer brakes requires air from the towing vehicle brake system in order to apply the trailer brakes. While that by itself does not involve any connection between the trailer electric system and the towing vehicle electric system, the presence of an ABS controller as part of the trailer air brake system does.
The industry has adopted certain standards for such receptacle sockets, and an example of one such standard is SAE (Society of Automotive Engineers) Standard J560. A receptacle socket compliant with that standard is sometimes referred to as a seven-pin, or seven-terminal, trailer socket, or connector. The standard specifies certain “pin-outs” for the seven individual terminals. Six terminals are arrayed in a circle at 60° intervals while the seventh is located at the center of the circle. Which circuits in the towing vehicle are connected to which terminals in the receptacle socket depends on the particular circuits in a particular trailer.
According to SAE Standard J560, the receptacle socket terminals are designated numerically in order, No. 1 through No. 7. Viewed axially in the direction looking at the open end of the receptacle socket, terminal No. 1 is at the 12:00 position, terminal No. 2 is at the 2:00 position, terminal No. 3 is at the 4:00 position, terminal No. 4 is at the 6:00 position, terminal No. 5 is at the 8:00 position, terminal No. 6 at the 10:00 position, and terminal 7 No. 7 is at the center.
The Standard specifies that: terminal No. 1 is a ground return from the trailer to the towing vehicle; terminal No. 2 is a feed for clearance, side marker, and identification lamps; terminal No. 3 is a feed for a left turn signal; terminal No. 4 is a feed for a stop signal; terminal No. 5 is a feed for a right turn signal; and terminal No. 6 is a feed for tail and license plate lamps. When a trailer is equipped with electric trailer brakes, terminal No. 7 is a feed for electric current that actuates the trailer brakes. When a trailer is equipped with air brakes, terminal No. 7 may not necessarily be used; however, when an ABS controller is associated with the trailer air brakes to endow them with ABS capability, terminal No. 7 provides a continuous D.C. power supply voltage for the ABS controller.
Hence, the Standard recognizes two distinct possibilities for trailer electric systems based on type of trailer brake system.
There are also two distinct possibilities for stop and turn signal lamp circuits in a trailer: 1) circuits that serve separate stop and turn signal lamps; and 2) circuits that serve combination stop and turn signal lamps.
A trailer that has separate stop and turn signal lamps has: 1) at least one right stop lamp, or lamp assembly; 2) at least one left stop lamp, or lamp assembly; 3) at least one right turn signal lamp, or lamp assembly; and 4) at least one left turn signal lamp, or lamp assembly.
A trailer that has combination stop and turn signal lamps has: 1) at least one right combination stop/turn signal lamp, or lamp assembly; and 2) at least one left stop/turn signal lamp, or lamp assembly.
These possibilities for different stop/turn signal lamps and different brakes lead to four possible circuit configurations for a trailer.
Configuration No. 1: Electric trailer brakes and combination stop and turn signal lamps.
Configuration No. 2: Electric trailer brakes and separate stop and turn signal lamps.
Configuration No. 3: Air trailer brakes and separate stop and turn signal lamps.
Configuration No. 4: Air trailer brakes and combination stop and turn signal lamps.
The receptacle socket in a towing vehicle is typically hard-wired for a particular one of these four configurations, enabling the vehicle to tow a trailer having that particular configuration. If a towing vehicle were to tow a vehicle having a different configuration, it has been the practice to provide an additional receptacle socket that is properly hard-wired for that different configuration. Hence, towing vehicles may have multiple receptacle sockets, each for properly connecting the particular electric system in a trailer to the towing vehicle electric system.
Certain passenger vehicles (many domestic-built) have combination stop and turn signal lamps while others (many non-domestic-built) have separate stop and turn signals. Domestic laws and regulations typically mandate that stop lamps be red. Consequently, combination stop and turn signal lamps in a motor vehicle will illuminate red whenever the driver is applying the brakes. Should the driver give a turn signal while applying the brakes, the lamp toward the side of the turn will flash red, while the opposite lamp remains continuously on as long as the brakes continue to be applied. Should the driver give a turn signal while not applying the brakes, the lamp toward the side of the turn will flash red, while the opposite lamp remains off. Combination stop and turn signal lamps in a trailer can simply be connected in parallel with the combination stop and turn signal lamps in the towing vehicle via the mated connector plug and receptacle socket.
The operation of stop lamps in a motor vehicle having separate stop and turn separate stop lamps is obviously independent of operation of the turn signal lamps, and vice versa. Separate stop and turn signal lamps in a trailer towed by such a vehicle can simply be connected in parallel with the separate stop and turn signal lamps in the vehicle via the mated connector plug and receptacle socket so that their operation is essentially slaved to that of the stop and turn signal lamps in the vehicle.
The rear turn signal lamps in a vehicle that has separate stop and turn signal lamps may have a color other than red, amber being an example of such an alternate color. Amber turn signal lamps may also be used in trailers having separate stop and turn signal lamps. So long as the towing vehicle has separate stop and turn signal lamps, the particular color of the separate trailer turn signal lamps is unimportant. That is not the case however when the towing vehicle has combination stop and turn signal lamps.
Were a receptacle socket in a vehicle having combination stop and turn signal lamps to be used to feed the stop and turn signal lamps in a trailer having separate stop and turn signal lamps, the trailer turn signal lamps would illuminate concurrently with the stop lamps. But should the trailer turn signal lamps be amber, a color that by itself is permissible under certain government regulations for signaling a turn, the unintended happenstance of amber turn signal lamps also signaling a stop may be impermissible, and hence should be avoided.
Were a receptacle socket in a vehicle having separate stop and turn signal lamps to be used to feed the stop and turn signal lamps in a trailer having combination stop and turn signal lamps, the trailer stop lamps would not illuminate concurrently with the stop lamps in the vehicle. That would also be impermissible, and hence should be avoided.